Joe85 Posted June 6, 2018 Share Posted June 6, 2018 Glorious Math guru's, please shower me with your superior intellect. A ball is thrown down at 20ms^-1 speed from the top of a building. The building is 125 meters tall. The distance travelled before it reaches the ground is as follows:S=U0t + 1/2gt^2U0 = 20ms^-1g = 10ms^-2t= timea) Find the time for the ball to drop a fifth of the height of the buildingb) Find the time for the ball to reach the groundNow, i feel fairly confident that i have solved this myself. Having converted this into a quadratic equation i have:a) t = 1sB) t = 3.385sHowever, when checking my answer against an online caluclator (https://www.calculatorsoup.com/calculators/physics/displacement_v_a_t.php) it states the answer is a complex conjugate. Now, i'm struggling to see how they have come up with a complex number here (i thought for it to be a complex number, the number you find the square root of (#^2- 4ac) had to be negative ie J = -1) but part of the course i am doing does involve complex numbers and i'm slightly worried they are trying to catch you out a bit.Any help for a dunce would be appreciated. Quote Link to comment Share on other sites More sharing options...
megawatt Posted June 6, 2018 Share Posted June 6, 2018 I may have answered this if it had said Chinese 125 dropped from top of building. But it doesn't, so I don't care. Quote Link to comment Share on other sites More sharing options...
Joe85 Posted June 6, 2018 Author Share Posted June 6, 2018 I may have answered this if it had said Chinese 125 dropped from top of building. But it doesn't, so I don't care. Noted for next time glorious mathematician. Quote Link to comment Share on other sites More sharing options...
Mr Fro Posted June 6, 2018 Share Posted June 6, 2018 I got the same answers as you. Quote Link to comment Share on other sites More sharing options...
Westbeef Posted June 6, 2018 Share Posted June 6, 2018 Ignore them calculators if they give you a quadratic answer and you’re not expecting it, they go mental sometimes. Quote Link to comment Share on other sites More sharing options...
bonio Posted June 6, 2018 Share Posted June 6, 2018 I think there's an error in the calculator.If I put the speed and acceleration as negative values, then it gives the right answerBut, putting your numbers for part a (with t = 1) into the equation s = ut + 1/2gt^2 you get25 = 20 + 0.5*10which is correct.My guess is that the person writing the page confused themselves by writing the equation as ut + 1/2gt^2 - s = 0, which led them to messed up the signs. Quote Link to comment Share on other sites More sharing options...
fastbob Posted June 6, 2018 Share Posted June 6, 2018 FORTY TWO ! Quote Link to comment Share on other sites More sharing options...
Bender Posted June 7, 2018 Share Posted June 7, 2018 I blame the mice. Quote Link to comment Share on other sites More sharing options...
TimR Posted June 7, 2018 Share Posted June 7, 2018 My answer is .Change the values to the following ...https://www.justanswer.com/math-homework/6ssh9-ball-thrown-down-72-km-h-1-speed-top.html Quote Link to comment Share on other sites More sharing options...
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